The invention relates to a heat exchange between a gas and falling pulverulent matter.
A common industrial operation entails recovery of heat from the combustion of a fuel or of xe2x80x9cwaste heatxe2x80x9d from a chemical process. Such recovery often entails the cooling of a hot gas against water, the water being either heated or converted to steam. Conventional equipment for cooling a gas is often large in size, because coefficients of heat transfer from a gas to a metal surface, in general, are relatively small, e.g., only a few tens of watts/m2xe2x88x92C. Achieving a high coefficient of heat transfer entails acceptance of a high pressure drop in the gas to be cooled. In practice, a balance must be struck between the capital expense for providing a larger heat exchanger and the running cost of a smaller exchanger, requiring higher pressure drop necessary in smaller equipment for it to perform the desired heat exchange.
Often, gas to be cooled is dirty, and in some instances, the dirt has properties causing it to foul heat-transfer surfaces with which the gas comes into contact. A notorious example is the off-gas from an electrometallurgical procedure for making ferrosilicon. This gas, as it enters a waste-heat boiler, contains an exceedingly fine fume of silicon dioxide, which fouls boiler surface so rapidly that a practice is to subject the surface to a shower of ball bearings every few minutes, cleaning it of adhering fume particles; yet even with this expedient, a larger boiler surface must be provided than would be necessary for cooling a clean gas. In some instances, a gas to be cooled contains a corrosive chemical species (such as hydrogen chloride), harmful to metal surfaces and over time reducing their effectiveness for transferring heat. Another notorious example arises in the manufacture of a fine titanium dioxide powder by burning titanium tetrachloride. It is difficult to maintain a reasonably continuous operation of the enormous xe2x80x9ctrombonexe2x80x9d heat-exchanger now used for cooling products of this combustion.
Heterogeneously catalyzed reactions, in general, are carried out either in fixed beds of a granular catalyst or in fluidized beds of a catalyst powder. In the latter, control of reaction temperature is relatively easy, since coefficients of heat transfer from a fluid bed to surfaces embedded therein are generally high, often in the hundreds of watts/m2xe2x88x92C. If, however, outcomes of a reaction are highly sensitive to axial gas dispersion (see Tshabalala and Squires, AIChE Journal, vol. 42, pp. 2941-2947, 1996), a fluid bed may not be a good choice. If a fixed bed must be specified, either a low coefficient of heat transfer from the reaction to surfaces within the bed must be accepted or a designer must adopt other expedients for controlling the bed temperature, such as employing a large gas recycle or injecting cold gas at intervals along the bed.
Herein, by the term xe2x80x9cvibrated bed,xe2x80x9d I mean a bed of powder in a chamber with a floor, this floor being vibrated vertically at a vibrational intensity sufficient to cause the powder to display the xe2x80x9ccoherent-condensed vibrated-bed statexe2x80x9d (see Thomas, Mason, Liu, and Squires, Powder Technology, vol. 57, pp. 267-280, 1989). In this xe2x80x9cstate,xe2x80x9d the powder becomes highly fluid. For example, application of only a small force is needed to move a stirring rod introduced into a vibrated bed from side to side. In general, intense vibration of a powder bed deeper than xcx9c1 mm causes the powder to enter the coherent-condensed vibrated-bed state.
I now provide a definition of xe2x80x9cvibrational intensity.xe2x80x9d I take the xe2x80x9cnull positionxe2x80x9d of the aforementioned floor to be its elevation when at rest. When it is subjected to a vertical sinusoidal vibration, its vertical displacement xcex6 from its null position is given by xcex6=xcex10 sin {overscore (xcfx89)}t, where xcex10=the maximum displacement (called xe2x80x9camplitudexe2x80x9d in the terminology of vibrated-bed engineering art); {overscore ({dot over (xcfx89)})}=2xcfx80ƒ; t=time; and ƒ=frequency. Vibrational intensity is the ratio of the floor""s maximum acceleration to the acceleration of gravity, and is given by xcex10{overscore (xcfx89)}2/g. For coarse powders, the threshold vibrational intensity for creation of a vibrated bed is a little greater than 1.0; for fine powders, the theshold intensity can be considerably higher than 1.0. (See Thomas, Mason, Liu, and Squires, 1989.) In commercial practice, vibrational intensities greatly exceed these thresholds. Intensities as high as 15 are commonly used.
Industry employs vibrated beds extensively for drying particulate material. The beds are sometimes large, e.g., several meters in width and ten or more meters in length. Heat of drying is sometimes provided by indirect heat transfer across heat-exchange surface positioned within the drying bed. A vibrated bed presents coefficients of heat transfer comparable to those afforded by fluid beds (see Thomas, Mason, Sprung, Liu, and Squires, Powder Technology, vol. 99, pp. 293-301, 1998). Accordingly, the quantity of heat-exchange surface required for indirect heat transfer in a vibrated bed drier can be small. In other vibrated-bed driers, heat of drying is provided by direct heat transfer from a hot gas introduced into the bed from below (thereby creating an xe2x80x9caerated vibrated bedxe2x80x9d).
Little power is required for vibrating a vibrated-bed drier if it is spring-mounted and vibrated at a natural frequency of its mount. An aerated vibrated bed for drying a relatively coarse pulverulent solid can often require far less power than a fluid bed for drying the same solid. The velocity of hot gas across the aerated vibrated bed can be small relative to the velocity necessary to fluidize the coarse solid, and so power required for gas compression can be far below that needed to supply hot fluidizing gas to a fluid-bed drier for the same solid. Power required for vibration can be as little as 10% of that which a fluid-bed drier requires for gas compression.
The high heat-transfer coefficients generally afforded by vibrated beds make them, in principle, attractive candidate devices for heat-exchange applications other than for drying particulate materials. As a practical matter, how to use a vibrated bed for recovery of heat from a hot gas, for example, is not obvious. Contemplating use of a vibrated bed for this application, I hoped to develop a heat exchanger in which hot gas would flow horizontally across the surface of a bed in which heat-exchange surface is embedded (see Sprung, Thomas, Liu, and Squires, in Fluidization V, edited by V. K. Ostergaard, Engineering Foundation, New York, 1986, pp. 409-416). With proper choice of particle size and vibration parameters, the surface of the bed would be dilute (i.e., surface powder would display the diffuse xe2x80x9ccoherent-expanded vibrated-bed statexe2x80x9dxe2x80x94see Thomas, Mason, Liu, and Squires, 1989). I hoped for an effective exchange of heat from the hot gas to the diffuse surface of the bed; or, failing that, I hoped that obliging the gas to flow through constrictions created by vertical baffles extending from the ceiling nearly to the bed""s surface would cause a sufficient quantity of powder to become entrained in the gas, thereby cooling the gas. Unfortunately, the coefficient for transfer of heat from horizontally flowing gas to a vibrated-bed surface was disappointingly small; and, as well, providing the vertical baffles did not sufficiently improve the rate of heat transfer at an acceptable pressure drop in the gas.
My invention overcomes the shortcomings of my aforementioned idea.
In the invention, a duct of substantially rectangular cross-section houses a vibrated bed of a pulverulent material and a superjacent space. The invention employs the vibratory motion that creates the vibrated bed to lift pulverulent matter from the bed in a continuous flow to substantially the elevation of the ceiling of the duct. The invention also employs the vibratory motion to distribute the matter across the duct""s ceiling, where the matter is allowed to fall into the aforementioned space. A gas is caused to flow horizontally along the space, the temperature of the gas being different from that of the pulverulent matter and, accordingly, exchanging heat therewith.
Two methods are available for employing the vibratory motion that creates the vibrated bed to lift the pulverulent matter to the elevation of the duct""s ceiling. One method is useful if the pulverulent matter, generally speaking, is a fine powder. A second method is useful if the pulverulent matter is coarse. Below, I will specify more closely what I mean by the terms xe2x80x9cfine powderxe2x80x9d and xe2x80x9ccoarse powder.xe2x80x9d
I now describe the two methods in turn.
The Fine Powder Method
I have discovered that a spout of particles emerges spontaneously from a small-bore tube positioned vertically in a vibrated bed of a fine powder and extending from an elevation close to the floor of the chamber housing the bed to an elevation comparable to that of the surface of the bed (Thomas, Mason, and Squires, Powder Technology, vol. 111, pp. 34-49, September 2000).
I do not fully understand this new phenomenon, but I associate it with the variation in floor pressure, beneath a vibrated bed, that accompanies a vibration cycle. Early in a sinusoidal vibration cycle (that is to say, at a relatively low phase angle), the floor pressure falls below the ambient pressure at the bed""s surface. Later (at a relatively high phase angle), the floor pressure exceeds the ambient. In general, the maximum positive deviation from floor pressure, late in the cycle, is much greater than the negative deviation, early in the cycle. More particularly, I associate the spouting phenomenon with the maximum positive pressure deviation occurring late in a vibration cycle. This deviation is a function of many variables, including vibrational intensity, density of the powder, depth of the powder bed, properties of the ambient gas, and (especially) the size of the powder. At the floor of a vibrated bed of a fine powder, the pressure deviation can be many times larger than it is for a coarse powder (see Thomas, Liu, Chan, and Squires, Powder Technology, vol. 52, pp. 77-92, 1987; Thomas and Squires, Physical Review Letters, vol. 81, pp. 574-577, 1998; and Thomas and Squires, Powder Technology, vol. 100, pp. 200-210, 1998). The discovery of spouting came about when a (conveniently at hand) small-bore tube was used to stir a vibrated bed of a fine powder, rather than a solid rod. The tube had a bore of 6 mm; its length was xcx9c15 cm. At a vibrational intensity of xcx9c6, the powder spouted to a distance amounting to a significant fraction of a meter. The powder was xcx9c70 micrometers in size and had bulk and intrinsic densities of 740 and 2,570 kilograms/m3, respectively. Spouts of this powder were seen at bed depths as small as xcx9c2 cm. Whether a bed of a given particulate will spout is a function of tube bore and length as well as the aforementioned variables that influence the maximum positive pressure deviation. I have not been able to explore a sufficiently wide range of the variables to provide a guide for predicting when a spout will form and when it will not. From equipment limitations, I have not been able to study vibrational intensities higher than xcx9c7 or beds deeper than xcx9c5 cm. Large-scale commercial vibration equipment is available for vibrational intensities as high as 15.0. If such a high intensity is employed and if a bed deeper than 5 cm is specified, it is probable that useful spouts can be created over a wide range of the remaining relevant parameters. In the practice of my invention, experimentation on a small scale can readily determine whether a spout will form under whatever set of conditions is of interest.
Herein, by the term xe2x80x9cfine powder,xe2x80x9d I mean a powder from which a useful spout can be created. In general, this will be a Group A powder in the Geldart classification (see Squires, Kwauk, and Avidan, Science, vol. 230, pp. 1329-1337, 1985).
The spouts of my discovery can be employed for lifting a fine powder from a vibrated bed of the invention to substantially the elevation of the ceiling of the duct. The vibration that created the vibrated bed is employed to create the spouts. This vibration is also employed to disperse powder transversely across the ceiling. For the latter objective, the vibrational intensity should be such that a spout colliding with the ceiling has a momentum sufficient to create a xe2x80x9ccloudxe2x80x9d of powder directly beneath the ceiling, within which powder moves laterally, and from which the powder is allowed to fall into the space.
The surface of a vibrated bed of a fine powder often displays heaps and depressions. The term of engineering art for the phenomenon is xe2x80x9cbunkeringxe2x80x9d; the bed is said to xe2x80x9cbunker.xe2x80x9d Desirably, however, a vibrated bed for practice of my invention should not bunker excessively. A downward flow of powder surrounds each spouting tube, and such flows mitigate against bunkering. In practice, providing a sufficient number of tubes should avoid excessive bunkering.
The Coarse Powder Method
Herein, by the term xe2x80x9ccoarse powderxe2x80x9d I mean a powder that will not form a useful spout as described above. In general, this will be a Group B or D powder in the Geldart classification (see Squires, Kwauk, and Avidan, op. cit.).
A fact well known to practitioners of vibrated bed art is that powder tends to move upward along a side wall if the wall diverges outward toward higher elevations. See, for example, H. Takahashi, A. Suzuki, and T. Tanaka, Powder Technology, vol. 2, pages 65-71 (1968/69). A vertical lift conveyor (U.S. Pat. No. 3,850,288, Nov. 26, 1975) employs the phenomenon. Herein, I use the term xe2x80x9cvibratory liftxe2x80x9d to signify a powder lifting device of this general nature. A vibratory lift can be used to elevate a relatively coarse powder to a height comparable to that of the ceiling of the substantially horizontal duct hereinbefore described. The vibration can also be employed in various ways to spread the powder across the ceiling of the duct, wherein openings allow the powder to fall into the space above the vibrated bed of the invention.
Bunkering is less of a problem for a coarse powder than it is for a fine powder.
Commercially available, nearly spherical particles of a crude alumina (designated xe2x80x9cMaster Beadsxe2x80x9d by the manufacturer, Norton-Alcoa) are advantageously employed as the coarse powder in some applications of the invention. They are highly resistant to breakage, reducing their size, or to attrition, producing a fine powder. They are available in several sizes.
Advantages of the Invention
In conventional boiler plant raising steam for generation of power, fans (induced-draft) consume a significant fraction of the power generated. The fraction often approaches 5% of the power. Responsible for this loss of power is the pressure drop through a convective heat-exchanger conventionally used (following a radiative heat-recovery section of a boiler) to recover heat from combustion off-gas. A significant advantage of my invention is the exceptionally small pressure drop that gas will experience when exchanging heat with the falling powder.
Sometimes a need arises to recover heat from a hot gas containing a corrosive chemical species, such as hydrogen chloride. An embodiment of my invention can provide protection of metal heat-transfer surfaces from substantial exposure to this species. In general, there is an in-and-out traffic of gas across the surface of a vibrated bed. Early in each (sinusoidal) vibration cycle, ambient gas enters the bed and causes it to expand. In a relatively shallow bed (e.g., in general, shallower than xcx9c25 cm), the gas penetrates all the way to the vibrating floor. (In beds of a relatively coarse powder, in general, the flow of ambient gas into the bed supplies gas for the formation of a xe2x80x9cgapxe2x80x9d between the floor and a xe2x80x9cbottom surfacexe2x80x9d of the powder.) Later in the cycle, this gas leaves the bed. (In the aforementioned beds of the relatively coarse powder, the gap closes late in the cycle.) This in-and-out flow of ambient gas, however, can be prevented by xe2x80x9caeratingxe2x80x9d the bed, i.e., by causing gas to flow into the bed at a sufficient rate from a plenum beneath the vibrating floor. In my alternative embodiment, I introduce a non-corrosive gas (such as air) into the vibrated bed of my invention via tubes extending from such a plenum to a mid-elevation within the bed. With these tubes so disposed, the lower levels of the vibrated bed act as a non-aerated bed, ensuring (in the fine-powder embodiment) the production of spouts from the small-bore, vertical tubes, while aeration of the bed""s upper levels substantially prevents penetration of the bed by the corrosive species.
An object of the invention is to provide a compact, non-fouling, long-lived, easily maintained heat exchanger for recovery of heat from a hot gas containing fume or dust or a corrosive chemical species.
Another object is to recover heat from a hot gas while causing only a small loss of pressure in the gas.
Another object is to provide a heat exchanger for control of temperature in a powdered catalyst promoting a chemical reaction.
Another object is to heat a powder.
Another object is to exchange heat between two gas streams.
Yet another object is to exchange heat from gaseous products of a combustion step to the oxygen-containing gas to be provided to this step, thereby heating this gas.
The invention relates to a heat exchange between a gas and falling pulverulent matter. The invention also relates to a double transfer of heat: a first exchange occurs between a gas and the falling pulverulent-matter; a second (transferring substantially the same quantity of heat) occurs between the matter and either a liquid or a second gas. The invention employs a coherent-condensed vibrated bed occupying the lower portion of a duct of generally rectangular cross-section. Gas flows in the horizontal direction through a space above the bed. Matter is conveyed from the bed to the elevation of the ceiling of the space, is distributed horizontally across the ceiling, and allowed to fall into the space. One object of the double heat exchange is to transfer heat from a hot gas to water. The gas may be a hot gas from combustion of a fuel. Alternatively, the gas may comprise chemical species capable of entering into a certain chemical reaction, the object of the exchange being to maintain a temperature favorable for this reaction in a space within which a catalyst is present with power to promote the reaction. In another alternative, a hot gas from combustion of a fuel may be cooled against a flow of air to be supplied to the combustion.
My invention relates to an improved method for exchanging heat between a gas and a pulverulent matter. The matter is introduced into a chamber having a substantially horizontal floor, a ceiling, a front wall, a back wall, and two side walls. The volume of the matter within the chamber is maintained at a volume that is significantly smaller than the volume of the chamber. Substantially vertical vibration is imparted to the chamber at a vibrational intensity sufficient to cause the matter to enter the coherent-condensed vibrated-bed state, thereby creating a vibrated bed that occupies a lower part of the chamber and a space that extends from the surface of the bed to the ceiling of the chamber. Matter is withdrawn from the vibrated bed and, through employment of the vibration, is elevated to substantially the elevation of the ceiling. Also through employment of the vibration, the elevated matter is distributed transversely across the ceiling. The distributed matter is permitted to fall through the aforementioned space. A gas is caused to enter the space across the front wall, to flow horizontally through the space, and to exit the space across the back wall, the gas having a temperature different from the falling matter and exchanging heat therewith.
My invention also relates to improved apparatus for the exchange of heat between a gas and a pulverulent matter. Means are provided for introducing matter into a chamber having a substantially horizontal floor, a ceiling, a front wall, a back wall, and two side walls. Means are provided for maintaining within the chamber a volume of the matter that is significantly smaller than the volume of the chamber. Means are provided for imparting substantially vertical vibration to the chamber at a vibrational intensity sufficient to cause the matter to enter the coherent-condensed vibrated-bed state, thereby creating a vibrated bed of the matter that occupies a lower part of the chamber and also creating a space that extends from the surface of the bed to the ceiling of the chamber. Means are provided for withdrawing the matter from the vibrated bed. Means are provided, through employment of the vibration creating the vibrated bed, for elevating the withdrawn matter to substantially the elevation of the ceiling. Means are also provided employing the vibration for distributing the elevated matter transversely across the ceiling. The distributed matter is permitted to fall through the aforementioned space. Means are provided for causing a gas to enter the space across the front wall, to flow horizontally through the space, and to exit the space across the back wall, the gas having a temperature different from the falling matter and exchanging heat therewith.